Crystalline silicon solar cells remain one of the most popular products in the photovoltaic industry. While other low cost alternatives have arisen, the alternatives have thus far provided low efficiency products. Industrial trends have increased demand for thinner wafers. Common industrial silicon solar cells suffer from a high rear surface recombination and a poor internal reflection of the full area Al back surface field (BSF). In 1989, Blackers et al. introduced a passivated emitter and rear cell (PERC) solar cell. Combining low surface recombination with a high internal reflection, PERC enables significantly higher conversion efficiencies compared to the A1-BSF technology.
It would be highly desirable to have a low cost but highly effective film coating process for the industrial fabrication of PERC solar cells. For the rear surface, a thick SiOx layer (˜100 nm) is required to obtain good passivation and internal reflection. For the front surface, either of single layer of SiNx or SiOx/SiNx stack (˜10 nm thin SiOx interlayer is inserted between the silicon surface and SiNx layer) is desired for both passivation and anti-reflectance purpose. Improved silicon surface passivation of SiOx/SiNx stacks over a single layer of SiNx has already been reported by a number of papers.
In addition, the recent progress of black silicon solar cell showed that applying 20 nm thermal SiOx passivation layer to a black silicon surface (a chemically etched porous Si surface that could drive the surface reflection down to 2˜3% across the spectrum of sunlight) could potentially replace the PECVD SiNx coated Si surface, resulting in low cost crystalline Si solar cell with minimized efficiency loss. The potential integration of the black silicon surface into PERC solar cell fabrication may increase the need for a coating process allowing the simultaneous growth of a thin film of SiOx on the front surface and a thick film of SiOx on rear surface in a single deposition step.
However, to coat a thin SiOx film on front surface and a thick SiOx film on rear surface of the silicon solar cell respectively in one growth step is difficult for commonly used thermal oxidation system. Alternatively, this is often achieved in two steps of coating process. First, a thin SiOx film (˜10 to 20 nm) is grown on both surfaces with thermal oxidation. Second, a SiOx thick film (˜100 nm) is deposited on the rear surface by the plasma enhanced chemical vapor deposition (PECVD). This two-step growth process increase the cost of production and increases production time significantly. Therefore, a new process that enables SiOx deposition on front and back surfaces with different thickness in a one growth process would be highly desirable.